Control of an ion energy distribution (IED) in a plasma is sometimes used to control substrate processes, such as selective etch processes or other suitable plasma processes. Control of the ion energy distribution in a plasma has been achieved by varying the amplitude or frequency of an applied radio frequency (RF) signal to control the width and average energy of the IED. For example, increasing the frequency of the RF signal can be used to decrease the width of the IED. However, the inventors have discovered that the wavelength at high frequency may become comparable to critical dimensions of features on the substrate which can result in process non-uniformities. Further, high frequency RF signals may not be compatible with components of a process chamber, such as an electrostatic chuck, which may be part of a substrate support.
Accordingly, the inventors have provided improved methods and apparatus for controlling the ion energy distribution of a plasma.